Silicones occupy an important place in the chemical industry in a variety of applications ranging from lubricants to sealants. Silicone fluids composed of polydimethylsiloxane combine many useful properties such as excellent thermal stability, chemical inertness, small changes in viscosity with changes in temperature, high oxygen permeability, outstanding shear stability, and excellent lubricity. They have found extensive use as adhesives, sealants, coatings, softeners, lubricants, dispersants, and antifoam agents in non-aqueous systems in industries as diverse as construction, pharmaceuticals, rubber, plastics, textiles, cosmetics, printing and founding.
More recently silicone rubber materials have been used in biomedical products such as artificial organs and contact lenses. The great benefit of silicone to the latter stems from its high permeability to oxygen although this advantage is offset by its intrinsic hydrophobicity. For this reason all present commercial contact lens formulations which incorporate silicone are hard lens formulations which contain it only in limited amounts. It would be highly desirable to produce a contact lens composed entirely of silicone rubber since it offers the highest oxygen permeability of all presently available materials. The enhancement of eye comfort and wearing time would represent a significant improvement in contact lens technology. However, this goal cannot be achieved until the hydrophobic surface of the silicone is rendered hydrophilic by some kind of surface modification, since silicone rubber alone adheres very strongly to the cornea and also rapidly attracts particulate matter.
Silicone rubber is made by crosslinking liquid polysiloxanes. These rubbers are thermoset polymers which can be made to varying degrees of hardness and elasticity by controlling the degree of crosslinking and the molecular weight of the silicone fluid. Silicone rubber is usually made by vulcanizing polydimethylsiloxane fluids with organic peroxides at elevated temperatures. Another approach to crosslinking employs hydrosilation in which poly(vinylalkylsiloxanes) are cured with poly(hydridoalkylsiloxanes) in the presence of transition metal catalysts. Silicone rubber has also been formed by chemically reacting, .alpha.,.omega.-difunctional poly(diorganosiloxanes) with polyfunctional silanes and siloxanes. Typically the crosslinking reaction is a condensation which forms a siloxane bond and a volatile by product. Common examples of this type of cure reaction are silanolalkoxylsilane (French Pat. No. 1,042,019), silanol-acetoxysilane (L. F. Ceyzeriat and P. Dumonth, German Appl. No. 1,121,803), silanol-silane (Midland Silicones, Brit. Pat. No. 804,199), and silanol-silanol (via the corresponding acetone oxime) (E. Sweet, Dow Corning, Belg. Pat. No. 614,394. Suitable catalysts for these reactions are amines and carboxylic acid salts of Sn, Pb, Zn, Sb, Fe, Cd, Ba, Ca and Mn.
Organosiloxane isocyanates have been prepared (U.S. Pat. No. 3,179,622) which vulcanize when exposed to moisture. In these cases the isocyanate group is joined to the siloxane through an alkyl group, rendering it unhydrolyzable.
Relatively few reports exist for the curing of silicones via the reaction of a silanol with an isocyanate yielding an .theta.-silylurethane linkage. This is probably due to the well known hydrolytic instability of this bond. To our knowledge there are no existing reports of a silicone rubber crosslinked by O-silylurethane bonds that is resistant to hydrolytic decomposition.
The use of silanol terminated polydimethylsiloxanes has been reported in the modification of polysiloxanes with polyurethanes (Moretto, U.S. Pat. No. 4,202,807) to improve the mechanical properties of the former at elevated temperatures. No mention was made of the hydrolytic stability of the O-silylurethane bond in these materials. The only other mention of this bond in a polymeric material was made by Kaufman, Muller, and Wegchaupt (U.S. Pat. No. 4,292,423 in the preparation of organopolysiloxanes for coating purposes. However, when the claimed organopolysiloxane was prepared entirely through the reaction of siloxane groups with isocyanate groups (Example 1 of this patent), the coating was decomposed by atmospheric moisture and addition of 75% by weight of the polyurethane, Desmodur L, was necessary to produce a hydrolytically stable coating.
High hydrolytic susceptibility is a property characteristic of O-silylurethanes since they were first reported by Andrianov, Losev, and Astakhin (Proc. Akad. Sci., USSR, Sect. Chem 113, 247, [1957]) from the reaction of triethylsilanol and m-toluene-diisocyanate. These authors subsequently described their alcoholysis and hydrolysis (J. General Chem., USSR, 29,2665, [1959]). This behavior of O-silylurethanes has been summarized by Noll in "Chemistry and Technology of Silicones," Academic Press, 1968, pp 99-100.
It has now unexpectedly been discovered, that by reaction of essentially equivalent amounts of diisocyanate-capped poly-dialkylsiloxanepolyalkanols and poly-dimethylsiloxane-disilanols, a clear, flexible and elastic siloxane polyurethane rubber is obtained which is completely resistant to hydrolysis under physiological conditions, despite the fact that half of all urethane groups in the polymer contain the --Si--O--C-- bond. This novel polysiloxane-polyurethane is uniquely suited as a biocompatible, oxygen-permeable membrane or film and especially, as a crosslinked rubber, as a soft contact lens.
It has further been discovered that this polyurethane is uniquely suited to be grafted to polyvinylalcohol or hydroxyalkyl cellulose and thereby allows production of a composite material not only with excellent optical clarity and the characteristically high permeability to oxygen, but also with high wettability as well.